Fabric filter systems, also known as baghouses, are air pollution control devices that remove the particulates out of air or gas released from industrial plants, including power plants, steel mills, cement plants, chemical plants, mining operations, and food processing plants. Typical fabric filter systems are made up of one or more cells, wherein each cell includes a plurality of rows of bags that act as a filter medium. Particulate-laden gas or air enters the fabric filter and is drawn through the bags, either on the inside or the outside depending on the cleaning method, and accumulates to form a layer of dust known as filter cake. The filter cake itself acts as filtering element, with the particles making up the filter cake capturing incoming fine particles and adsorbing gas phase impurities that would otherwise not be caught by the bags themselves.
As the filter cake builds on the bag, it becomes more difficult for air to move through the bag, causing a pressure drop (or differential pressure) and a reduction in the ability of the bag to continue filtering particulates. Therefore, many systems monitor the pressure drop across the fabric filter and institute a periodical cleaning process each time the pressure drop reaches a certain level. The fabric filters themselves are often classified based on the cleaning method used, with the most common being mechanical shakers, reverse air, and pulse jet.
Mechanical shakers use shake cycles of vibrations to create waves in the bags, thereby removing the filter cake layer. However, during the shake cycle, there can be no positive pressure and therefore the bag or compartment of bags must be taken offline and isolated from the input air streams during cleaning.
In reverse air fabric filters, dirty air flow normally enters the fabric filter and passes through the bag from the inside causing filter cake to build up on the inside. Bags are cleaned by flowing clean air through the bag in a reverse direction as compared to the normal air flow. This reverse air flow pressurizes the bag, making the bags partially collapse and causing the filter cake to break apart and drop out of the bag. When the cleaning is complete, the reverse air flow is discontinued and normal input air flow is reintroduced into the bag. As with mechanical shaking, this process also requires that the bag or compartment of bags be taken offline and isolated from the input air.
Finally, in pulse-jet fabric filters, pulses of high pressure air are sent down the inside of the bag to remove the filter cake which accumulates on the outside of the bag. Typical pulse-jet fabric filter cleaning systems use fixed dwell pulse control timing boards to sequentially control the cleaning of fabric filter bags. Each cell in the fabric filter system is capable of being brought online/offline separately from the other cells. Traditionally, pulse-jet cleaning systems perform offline cleaning in a cycle wherein each cell is periodically isolated and pulsed down thoroughly. During the offline cleaning, overall differential pressure across the entire fabric filter is elevated while any cell is isolated, and when a cell is returned to service the filter cake layer on the surface of all bags in the cell is much thinner than the average or normal thickness. These systems completely clean one cell before moving on to the next, utilizing a fixed width dwell time between pulses and a fixed width pulse time. The dwell time width and pulse time width settings may be adjusted, such as by using a variable resistor in an RC circuit built into a pulse timing board. Typically, these settings are based on worst case filter cleaning demand, wherein the fixed settings are determined based on an assumption that the maximum amount of filter cake has been deposited on each cell within a certain period of time. This approach initiates cleaning as an on/off function of the sensed fabric filter differential pressure. These traditional pulse-jet cleaning systems use either a differential pressure switch or a value from a differential pressure transmitter to behave as a switch, turning the cleaning on and off when the differential pressure reaches high and low set points. When the differential pressure hits the high set point, cleaning is activated. During cleaning, the differential pressure falls and eventually reaches the low set point at which time the cleaning cycle is deactivated. Typically, there is some deadband value between the “on” and “off” commands, such that the cleaning cycle remains off until the pressure builds back to the high set point.
The traditional approach of taking individual cells of bags offline to perform cleaning leads to several problems. For example, the cleaned bags receive significantly more input air and gases as compared to cells of bags which were not recently cleaned because the input flow naturally diverts to bags with less filter cake providing less resistance to gas flow, causing unequal distribution of the input air and gases within the fabric filter. As noted above, traditional cleaning methods using fixed dwell times and pulse times may also remove too much of the filter cake, leaving a layer that is too thin to trap fine particulates. Instead, it is now seen as advantageous to have a system allowing online cleaning without taking cells out of service. In order to achieve online cleaning, the pulses must create sufficient air pressure within the bag to overcome the inflow of air and gas through the bag. However, existing systems implementing online cleaning still clean only one cell at a time, and still rely on sensed high and low pressure drop points to turn the pulse jets on and off. These lead to inconsistent differential pressure within the system, insufficient layers of remaining filter cake to trap fine particulates, and unequal distribution of inflow air and gas within the fabric filter.
What is needed is a system and method for continuous online cleaning with flexible cleaning sequences to distribute cleaning across the fabric filter, and to manage the cleaning sequences based on actual loading to maintain a consistent differential pressure, improving filtration.